Lasers as sources of electromagnetic radiation

Lasers – division depending on the active medium used

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. They work by amplifying the emitted light by forcing the emission. They emit electromagnetic radiation in the range of visible, ultraviolet or infrared light. The operation of lasers is based on stimulated emission, which consists in illuminating excited atoms with radiation of a defined energy.

The most general division of lasers is based on their classification depending on the active medium or the wavelength of the emitted radiation. Taking into account the active medium present in the laser, we can distinguish gas, liquid and solid-state lasers. Molecules, atoms or ions that are part of such a medium differ in their energy structure. It determines the most important parameters of the laser.

Below are the most important examples of lasers, depending on the active medium used. In parentheses are the wavelength ranges of the emitted wave:

Gas lasers

• Helium-neon (543 nm or 633 nm)
• Argan (458nm, 488nm or 514.5nm)
• Nitric (337.1 nm)
• Krypton (647.1 nm, 676.4 nm)
• Carbon dioxide (10.6 μm)

Liquid lasers

• Dye laser (400 nm – 700 nm)

Solid-state lasers

• Ruby laser (694.3 nm)
• YAG neodymium laser
• Glass neodymium laser
• YAG erbium laser (1645 nm)
• YAG thulium laser (2015 nm)

Characteristics of selected lasers

• Argon laser

The argon laser belongs to the group of gas ion lasers. The active medium in this case is formed by argon ions. This laser can emit more than 30 lines ranging from ultraviolet to red light. Argon atoms are held in the discharge tube at a pressure of about 0.1 Torr. The electrons created during the discharge collide with argon atoms. They can directly ionize and excite them, moving the atoms from the ground state to the excited ion state. Another, more effective process is the two-stage ionization of argon. The ion formed this way is then transferred to an even higher excitation state, which is called the upper laser state. This makes it possible to generate several spectral lines with different frequencies.

• Helium-neon laser

The helium-neon laser is an example of a gas laser, constructed in 1959. Light is emitted as a result of the so-called population inversion. Helium and neon are placed in a 10:1 ratio (total pressure is close to 1.3 hPa) in a quartz glass tube. Voltage is applied at its ends, which causes discharges in the gas. As a result, an electrostatic field is created inside the pipe. It accelerates electrons and ions to high speeds. Because there are more helium atoms inside such a laser, accelerated electrons hit them much more often and cause their excitation to higher energy states, which are relatively stable for a relatively long time. The excited helium atoms in turn collide with the neon atoms and transfer the excitation energy to them. For this gas, excitation times at a higher level are greater than at a lower level, therefore, after some time, the so-called population inversion occurs.

• Carbon dioxide laser (molecular)

Such lasers can operate in both continuous and pulse modes. The active medium in this case is a mixture of carbon dioxide (CO₂), nitrogen (N₂) and helium (He) in a volume ratio of 1: 1.3 : 1.7. Each of them fulfils specific functions. Carbon dioxide is the active gas, electric discharges, which provide excitation energy, take place in nitrogen, while helium is designed to stabilize the CO₂ plasma and dissipate the resulting heat. Electrical discharges that take place in a mixture of carbon dioxide and nitrogen cause a very effective excitation of N₂ molecules. Since such a molecule has identical nuclei, a dipole transition is forbidden. Energy is lost only as a result of collisions. If there are carbon dioxide molecules in the molecular laser tube, as a consequence of the good coincidence of the excited N₂ and CO₂ levels, collisions of the second type cause the excitation of CO₂ molecules and return to the ground state of N₂ molecules. In this case, inversion in the mixture is achieved much more easily than in pureCO₂.

• Ruby laser

It was constructed in 1960 by Theodore Maiman. The active substance responsible for the properties of the ruby laser is ruby (aluminium trioxide, Al₂O₃, in which some of the aluminium atoms are replaced by Cr³⁺ chromium atoms). Ruby lasers operate in pulses, emitting radiation in the visible red light range. The central part of the laser is a ruby rod with a flash lamp above it. The intense flash of light coming from it excites some ruby atoms to a higher energy state. In turn, the ruby atoms excite other atoms in this way by sending photons. On both sides of the ruby rod there are mirrors that enhance this effect. One of them is semi-permeable, and the photons that escape through it are the resulting laser beam. Ruby lasers are now mainly of historical interest. Their use is limited to holography or tattoo removal.